Opioids are substances that act on opioid receptors to produce morphine-like effects. Opioids are most often used medically to relieve pain. Opioids include opiates, an older term that refers to such drugs derived from opium, including morphine itself. Other opioids are semi-synthetic and synthetic drugs such as hydrocodone, oxycodone and fentanyl; antagonist drugs such as naloxone and endogenous peptides such as the endorphins.
Fentanyl is a μ-opioid receptor agonist with analgesic potency approximately 80-100 times that of morphine. In clinical settings, fentanyl exerts its principal pharmacologic effects on the central nervous system. Its primary actions are analgesic and sedation.
The analgesic effects of fentanyl are related to the blood level of the drug. In general, the minimum effective concentration and the concentration at which toxicity occurs rise with increasing tolerance to any and all opioids. The rate of development of tolerance may vary widely among individuals. All opioid mu-receptor agonists, including fentanyl, produce dose dependent respiratory depression. The risk of respiratory depression is typically less in patients receiving chronic opioid therapy who develop tolerance to respiratory depression and other opioid effects. Serious or fatal respiratory depression can occur, even at recommended doses, in vulnerable individuals.
Orally administered fentanyl is subject to first pass effect metabolism as upwards of 50% or more of orally administered fentanyl is not absorbed. Other forms of delivery such a parenteral, buccal, and transdermal have been utilized to decrease or avoid this first pass effect for fentanyl. Fentanyl is currently available in injectable form, as a lozenge (e.g. Actiq® (a registered trademark of Cephalon, Inc.)), as a transdermal system (e.g., Duragesic® (a registered trademark of Johnson & Johnson) 25, 50, 75, and 100 μg of fentanyl per hour) and a sublingual spray (e.g. Subsys®, a registered trademark of Insys Development Company, Inc.).
Naloxone has the following structure and is synthesized from thebaine:

Naloxone is most commonly used to treat patients suffering from opioid dependence or overdose because it is a competitive μ-opioid antagonist that blocks the effects of opioids. Naloxone is currently available in Suboxone® (Suboxone is a registered trademark of Reckitt Benckiser Healthcare (UK) Limited) as tablet or sublingual film strip formulations. Suboxone® contains buprenorphine and naloxone in a 4:1 ratio.
One issue with other opioid dependence treatments is that they can become addictive. Naloxone, however, does not appear to be addictive and patients do not build up a tolerance.
Naloxone has also been used as a treatment for cognitive insensitivity to pain with anhidrosis. Insensitivity to pain with cognitive anhidrosis is a disorder in which the patient cannot feel pain.
Naloxone may be administered orally, intravenously, by injection or via the nasal mucosa. Naloxone has a low mean serum half-life when administered parentally. The quick metabolism may require repeat dosing or cause patient discomfort between doses. Enteral administration has low bioavailability due to hepatic first pass metabolism.
Naloxone is often administered to counteract the effect of fentanyl, for example to revive a patient after a fentanyl overdose. However, there is a need in the art for effective sublingual compositions comprising both an opioid, preferably, fentanyl, and naloxone so that naloxone could mitigate the effects of an opioid yet not block the clinical effect of the opioid. There is also a need in the art for formulations that would be opioid abuse deterrents.